Patent document 1 (JP 2008-180596A, US 2008/0174450) discloses a PWM output type sensor circuit that outputs a PWM output signal, which has a pulse width associated with a value of a physical quantity, as information concerning the physical quantity.
Patent document 2 (JP H06-284100) discloses a technology implemented in a multiplex communication method, in which plural analog signals that should be transmitted are pulse-width modulated and then time-division multiplexed, the multiplexed pulse width-modulated signals are transmitted over a sole transmission line, the signals are received over the transmission line in order to detect synchronous information, and the plural pulse-width modulated signals are demultiplexed from the received signals on the basis of the detected synchronous information. According to the technology, while the timing of delivering the pulse-width modulated signal onto the transmission line is circulated among a greater number of timings than the number of analog signals that should be transmitted, the plural pulse-width modulated signals whose rising timings are controlled to be constant are delivered onto the transmission line at associated delivery timings, and a period during which no pulse-width modulated signal is delivered is detected as the synchronous information.
Patent document 3 (JP H07-7490) discloses a pulse multiplex transmission method. According to the pulse multiplex transmission method, in a transmitting unit, a first pulse width modulator sequentially multiplexes plural digital signals into a first pulse-width modulated signal stream, a second pulse width modulator sequentially multiplexes other plural digital signals into a second pulse-width modulated signal stream that has a reverse polarity of the polarity of the first pulse-width modulated signal stream, and a multiplexer multiplexes the first pulse-width modulated signal stream and second pulse-width modulated signal stream. In a receiving unit that receives a multiplexed pulsating signal produced by the transmitting unit, after an ac-coupled amplifier amplifies the multiplexed pulsating signal into a pulsating signal having a predetermined amplitude, a subtractor, a first pulse-width demodulator, and a second pulse-width demodulator sequentially separate the plural original digital signals from one another so as to reproduce the original digital signals.
Non-patent document 1 (“Surface Vehicle Information Report” from the Society of Automotive Engineers (SAE) International (J2716 Revised February 2008, SENT for Automotive Application)) discloses a single edge nibble transmission (SENT) communication technology for decreasing the number of communication pulses per unit time. Herein, since an output signal of a sensor is divided into plural pulsating outputs in units is of four bits, a pulse serving as a reference for correction is transmitted as a leading pulse, and a quantity to be corrected with the trailing pulse is supposed to be an estimate. When multiple data items are communicated, a message identification (ID) is assigned to each of the data items in order to treat the data items independently.
For communications among various kinds of sensors (for example, a pressure sensor, a temperature sensor, an optical sensor, and a slope sensor), which are mounted in a vehicle, and an on-board electronic control unit (ECU), signal lines are provided for the respective sensors.
However, in recent years, it is requested to decrease the number of signal lines by integrating plural sensors, and multiplex sensor output values (detection signals) of the plural sensors so as to communicate the sensor output values, and thus reduce a cost.
For example, when a pressure sensor output value detected by a pressure sensor is transmitted to the ECU, a change in a pressure is accompanied by a temperature change according to a gas state equation.
Therefore, it is requested to highly precisely implement various controls in a vehicle on the basis of an accurate pressure sensor output value. Specifically, the pressure sensor and a temperature sensor are integrated. When the pressure sensor is used to detect a pressure sensor output value, the temperature sensor is used to detect a temperature sensor output value at the same time. The pressure sensor output value and temperature sensor output value are multiplexed and communicated to the ECU, and the ECU corrects the pressure sensor output value on the basis of the temperature sensor output value.
In practice, a total of three wiring harnesses including a wiring harness that is an analog signal line over which analog signals of sensor output values are communicated, a wiring harness that is a positive power line, and a wiring harness that is a ground line are used to connect the sensors and ECU.
In the ECU, the analog signals of sensor output values are analog-to-digital converted. It is likely that the precision in digital signals produced through the analog-to-digital conversion may be degraded due to an adverse effect of noises convoluted to the ground line. Therefore, in the ECU, a digital filter is applied to the analog-to-digital converted signals in order to remove the noises.
However, when the number of data sampling times is increased in order to improve the noise removing performance of the digital filter in the ECU, the responsiveness of a sensor output value may be degraded.
After a sensor output value is converted into a digital signal, when the digital signal is transmitted to the ECU, the adverse effect of the noises can be avoided. However, high timer precision is needed in order to realize high-speed communication.
For example, when multiplex communication is performed by utilizing controller area network (CAN) communication, it is possible to correct timer precision. However, a circuit dedicated to the correction of the timer precision has to be included. The inclusion of the dedicated circuit increases costs.
The patent document 1 discloses a technology for convoluting information relevant to a value of a physical quantity to a PWM output signal by determining a crest value of the PWM output signal in relation to the value (temperature information) of the physical quantity other than a certain physical quantity (a battery current value).
According to the technology, a dedicated circuit has to be included in an ECU in order to detect the crest value of the PWM output signal. As a result, cost is increased due to the inclusion of the dedicated circuit.
For PWM communication between a sensor and an ECU, normally, an output comparison circuit is included in the sensor and an input capture circuit is included in the ECU. A timer clock is fed from different timer circuits to the output comparison circuit and input capture circuit respectively.
Therefore, even when the input capture circuit exhibits an excellent timer resolution, a variance occurs in the precision in a pulse width of a sensor output value due to a difference between the timer resolution of the input capture circuit and the timer resolution of the output comparison circuit. Restrictions are therefore imposed on an amount of data capable of being communicated. Therefore, the technology is not satisfactory for multiplex communication.
The timer resolution of the input capture circuit refers to a cycle of a timer clock of a timer circuit included in the input capture circuit. The timer resolution of the output comparison circuit refers to a cycle of a timer clock of a timer circuit included in the output comparison circuit.
A variance in the precision (timer precision) in the pulse width between the timer clocks produced by the respective timer circuits included in the input capture circuit and output comparison circuit respectively leads to a difference in the timer resolution between the input capture circuit and output comparison circuit. Therefore, the difference in the timer resolution refers to a timer-precision difference.
The technology in the patent document 2 addresses a problem that high-speed communication is hard to do because a communication time is extended by a time that is a sum of a time required to circulate the timing of delivering a pulse-width modulated signal onto a transmission line among a greater number of timings than the number of analog signals that should be transmitted, a time required to deliver plural pulse-width modulated signals, which have rising timings thereof controlled to be constant, onto the transmission line at the associated delivery timings, and a time required to send synchronous information.
The technology in the patent document 3 addresses a problem that a communication time is long and high-speed communication is hard to do. This is because since one pulse of a PWM signal is used to transmit or receive one bit of a digital signal, numerous pulses are needed to communicate all data items.
Another problem of the technology in the patent document 3 is that, since modulating or demodulating processing is performed using a pulse-width modulator or a pulse-width demodulator, an error is caused in the modulating or demodulating processing.
In addition, according to the technology in the patent document 3, since special circuits such as the pulse-width modulator and pulse-width demodulator and an ac-coupled amplifier have to be included, a cost is increased due to the inclusion of the special circuits.
The technology in the non-patent document 1 addresses a problem that, since numerous pulses are needed to communicate all data items, the communication time is long and high-speed communication is hard to do.
In addition, according to the technology in the non-patent document 1, since interrupt processing is performed for each pulse in an ECU, when high-speed communication is performed, the interrupt processing has to be performed frequently. Eventually, processing other than communication to be performed in an ECU is retarded.